1. Field of the Invention
The present invention pertains to spectrometers, and, more particularly, to a filter wheel and a spectrometer using the filter wheel.
2. Description of the Related Art
Gas analyzers such as spectrometers are widely used in medical applications to measure concentration of carbon dioxide, oxygen gas and anaesthesia agents such as halothane (2-bromo-2-chloro-1,1,1-trifluoroethane), enflurane (2-chloro-1,1,2,-trifluoroethyl-difluoromethyl ether), isoflurane (2-chloro-2-(difluoromethoxy)-1,1,1-trifluoro-ethane), sevoflurane (2,2,2-trifluoro-1-[trifluoromethyl]ethyl fluoromethyl ether), and desflurane (2,2,2-trifluoro-1-fluoroethyl-difluoromethyl ether) during patient anaesthesia. There are two main types of gas analyzers, gas analyzers that are located either in the main path of a patient's respiratory gases (main flow measuring gas analyzers or main stream gas analyzers) or lateral flow measuring gas analyzers. The lateral flow measuring analyzers take a sample from the respiratory circuit of a patient to an adjacent instrument wherein actual gas analysis is performed. On the other hand, main flow or mainstream measuring analyzers calculate gas concentration directly in the respiratory circuit of the patient. Typically, the main flow analyzer is positioned in close proximity of a patient's mouth or trachea. The main flow gas analyzers or spectrometers incorporate the optical and electronic components of the spectrometer in one housing. As a result, in a clinical setting, it is desirable that the mainstream gas analyzers be as compact and lightweight as possible.
Respiratory gas can be analyzed using different methods, including dispersive and non-dispersive spectroscopy. The most common method of gas analysis is based on non-dispersive spectroscopy wherein gases absorb radiation energy (e.g., infrared energy) at a wavelength specific to the gas of concern (e.g., carbon dioxide). A light beam from a light source (e.g., infrared light source) is passed through a patient's respiratory circuit. The light beam that passes through the respiratory circuit is absorbed at various spectral regions specific to the gas in the respiratory circuit of the patient. A detector assembly is positioned on an opposite side of the respiratory circuit of the patient. The detector assembly includes a detector for measuring light intensity and a bandpass filter. The bandpass filter is positioned such that the light beam will pass through the filter before reaching the detector. The bandpass filter can be selected to filter out undesired regions of the wavelength spectrum in the light beam and transmit a spectral region of interest corresponding to an absorption region of the gas in the respiratory circuit of the patient.
While conventional mainstream analyzers may operate well for a small number of specific, non-overlapping spectrum wavelengths (e.g. in the analysis of individual gases such as carbon dioxide), this type of system has had some limitations. For example, the conventional mainstream analyzers can become inefficient for the analysis of a plurality of gases wherein more than 2 or 3 wavelength regions may be involved requiring the use of a plurality of filters.